Carburization installation

ABSTRACT

An installation for heat treatment or thermochemical treatment of steel permit instantaneous and permanent checking of the concentration of carbon at the surface of the steel, particularly in the course of the carbon enrichment of the surface zone of a workpiece. The process provides for the circulation in contact with this surface zone of a gaseous flux permitting the saturation concentration of carbon at the surface to be obtained at maximum speed, and for regulating this flux as a function of time. The installation comprises a monitored-atmosphere furnace having an input and an outlet for a gaseous flux, feed means forming this gaseous flux, and means for regulating the gaseous flux; it is equipped with data-storage and/or calculating means, and the feed means are designed to form a gaseous flux suitable for obtaining a saturation concentration of carbon at the surface of the workpieces in less than one minute.

This is a continuation of application Ser. No. 07/894,350 filed Jun. 4,1992 now abandoned, which in turn is a division of application Ser. No.07/549,968, filed Jul. 9, 1990, now U.S. Pat. No. 5,139,584.

This invention relates to the carburization of steel, and moreparticularly to a process for heat or thermochemical treatment of asteel, of the type permitting instantaneous and continuous checking ofthe concentration of carbon at the surface of the steel, especially inthe course of the carbon enrichment of the surface zone of a workpiece.

The invention further relates to an installation for carbon enrichmentof the surface zone of steel workpieces, of the type comprising amonitored-atmosphere furnace having an input and an outlet for a gaseousflux, feed means forming this gaseous flux, and means for regulating thegaseous flux.

The checking of the concentration of carbon at the surface of steel inthe course of a heat treatment or of a thermochemical treatment whileavoiding undesirable complementary reactions, such as oxidation, forexample, is a problem of great industrial importance, the solution ofwhich calls for the implementation of multiple technological solutions.

In the case of heat treatment, the solution chosen for highly alloyedsteels, sensitive to oxidation, calls for vacuum technologies. In thecase of thermochemical treatments, of cementation or case-hardening, forexample, a gas-solid or liquid-solid reaction is utilized. Chemicalspecies present in the gaseous phase or in the liquid phase decompose atthe surface of the steel, releasing the carbon, which diffuses into thesolid.

In vacuum heat-treatment processes, the oxidation and thedecarburization of the steel are avoided by maintaining a very low totalpressure of the atmosphere of the furnace (less than 0.1 mb).

In the best-monitored case-hardening processes, the steel is placed incontact with an atmosphere in thermodynamic equilibrium, the feed ofwhich is sufficiently high so that the equilibrium is not appreciablymodified by the transfer of the carbon into the solid. In this case, thechemical species of the gaseous phase which serves as a support for thecarbon is carbon monoxide (CO). Its decomposition at the surface of thesteel releases oxygen, the partial pressure of which must be kept verylow.

The problem of the case-hardening of steel will now be analyzed indetail.

BACKGROUND OF THE INVENTION

The rates of cementation, i.e., the conditions for obtaining the profileof carbon concentration in the shortest possible time, depend upon therates of transfer of the chemical species in the liquid phase or in thegaseous phase (e.g., carbon monoxide in the gaseous phase), the rates oftransfer of the carbon at the liquid-solid or gas-solid interface, andthe diffusion of carbon in the solid phase.

In the conventional processes, the limiting stage is that correspondingto the transfer of the carbon to the interface, either because thedecomposition reaction of the chemical species is slow or because thedecomposition products of these species cause a resistance to thetransfer of the carbon at the surface of the steel.

Thus, for the conventional processes of case-hardening in the gaseousphase by carbon monoxide, the elimination of the oxygen is too low,which greatly reduces the flux of carbon transferred. This is shown inthat the surface concentration of carbon does not reach the maximumvalue, permitting the diffusion of the carbon into the solid at theoptimum rate, until quite some time, e.g., 20 min., after having beenput in contact with the appropriate atmosphere in the usual conditionsof temperature and pressure.

DESCRIPTION OF THE PRIOR ART

There is disclosed particularly in an article by J. Wunning et al.,entitled "Gesteuerte Aufkohlung," published in HTM 31, 1976 3, as wellas in German Patent No. 3,139,622, a process according to which certainparameters are measured during the carbon-enrichment phase and serve,after processing, for determining operational data of the installation.

However, the teaching contained in these references does not permitconducting a carbon-enrichment process practically in the minimumpossible time. Furthermore, the prior art process takes place at reducedpressure.

The research which led to the present invention came to the followingfindings:

In order for the transitory conditions to be as brief as possible, thecarbon flux transferred via the gaseous phase and to the gas-solidinterface must be greater than, then equal to, the carbon fluxtransferred in the solid phase by diffusion for a maximum carbon surfaceconcentration. For that purpose, there must be:

a carbon-vector chemical compound in the gaseous phase having very rapiddecomposition kinetics at the surface of the steel,

a reactor having thoroughly agitated reactor behavior,

a gas-solid reaction permitting any formation of a transfer resistanceat the surface of the steel to be avoided.

Such transfer resistance may consist either of a layer of adsorbedchemical elements resulting from the decomposition of the carbon-vectorchemical species, or of the formation of a continuous layer of aspecific compound (carbide) in which the diffusion of the carbon isslow.

If the appearance of this transfer resistance is avoided, if thedecomposition at the surface of the steel of the molecule ensuring thetransfer of the carbon in the gaseous phase is very rapid, if the feedof carbon at the input of the reactor is sufficient, and if the reactoris thoroughly agitated, there is a maximum flux of carbon transferredinto the steel, and the rate of elaboration of the gradient of carbonconcentration in the steel depends only on the diffusion of the carbonin the solid state.

When these conditions are satisfied, the surface carbon concentration ofa steel initially having a carbon concentration of 0.2% reaches itsoptimum value (a value at least equal to the saturation concentration ofthe austenite) in less than one minute, a period of time to be comparedwith the dozens of minutes necessary in the case of a conventionalcase-hardening process by carbon monoxide.

The treatment of cementation by the carbon is effected in this case atan optimum rate of speed.

It is therefore an object of this invention to define practical meanspermitting this result to be achieved industrially.

SUMMARY OF THE INVENTION

To this end, in the process according the present invention, of the typeinitially mentioned, there is caused to circulate in contact with thesurface zone a gaseous flux permitting the saturation concentration ofcarbon at the surface to be obtained at maximum speed, and this flux isregulated as a function of time.

The installation according to the present invention, also of the typeinitially mentioned, is equipped with data-storage and/or calculatingmeans, and in that the feed means are designed to form a gaseous fluxsuitable for obtaining a saturation concentration of carbon at thesurface of the workpieces in less than one minute.

These conditions are achieved by using a gaseous mixture which containsno oxygen, made up of a chemically inert support gas or gaseous mixture,of a gas or a gaseous mixture permitting any oxidation phenomenon to beavoided during the periods of the treatment cycle when the carbon fluxat the surface of the workpiece is nil, of a gas or a gaseous mixturepermitting the transfer of the carbon and made up of compounds free ofoxygen. The gas or the gaseous mixture permitting the transfer of thecarbon contains at least one chemical compound, the pyrolysis of whichat the surface of the steel releases carbon and leads to the formationof a mixture of chemically stable by-products in the gaseous phase, inthe conditions of temperature and of pressure considered. Thus, the fluxof carbon transferred can be controlled by the feed of the pyrolysablecompound introduced into the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred practical embodiment of the invention will now be describedin detail with reference to the accompanying drawings, in which:

FIG. 1 is an elevation of an installation for developing the basic dataof the implementation of the process,

FIG. 2 is a comparative graph of carbon enrichment carried out accordingto different methods,

FIGS. 3 and 5 are graphs illustrating embodiments of implementation ofthe process at two different temperatures,

FIG. 4 is a diagrammatic view of an installation for implementing theprocess, and

FIG. 6 is an auxiliary graph relating to the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In practice, the treatment may be carried out in the following manner:

a first phase (I) at the maximum possible temperature, defined on thebasis of metallurgical criteria, with control of the input feed ofhydrocarbons for obtaining a maximum concentration of surface carbon,this maximum concentration being at least equal to the carbon saturationconcentration of the austenite;

a second phase (II) at the maximum possible temperature, defined on thebasis of metallurgical criteria, during which the flux of carbontransferred from the gaseous phase is nil in order to adjust the carbonconcentration profile and, in particular, the surface concentration, toa predetermined value chosen for metallurgical reasons. In numerouscases, this value is close to 0.7/0.8% on conventional case-hardenedsteels.

At the end of this second phase, the temperature may be that chosen forcarrying out tempering of the steel.

It is possible to carry out a cycle by an appropriate succession ofphases I and phases II, the number and duration of which depend upon thedesired carbon concentration profile. In this case, however, the totalduration of the cycle will be longer than in the case of a cyclecomprising only one phase I and one phase II.

Phase I necessitates precise checking of the surface concentration ofcarbon based on the carbon flux consumed by the steel. This carbon fluxis in turn monitored by the feed of hydrocarbons introduced into thereactor. If the concentration of carbon at the surface of the steel isknown, the rate of carbon consumed is easily calculated. Conversely,continous checking of the rate of carbon consumed permits maintenance ofa constant surface carbon concentration. The rate of carbon consumed isotained by drawing up the material balance-sheet of the reactor, eitherafter preliminary identification of the different chemical speciesformed, as well as the evolution of their concentration, or by ananalysis at each moment of the gaseous mixture leaving the reactor. Thesecond solution avoids uncertainties of a physiochemical model, and itpermits checking the operation of the reactor and the evolution of thegas-solid reaction at each moment.

Flawless monitoring of the carbon surface concentration during phase Iis indispensable for an accurate knowledge of the profile of carbonconcentration in the steel at the end of this phase. Indeed, it is onthe basis of this profile that the duration of phase II will bedetermined for obtaining, at the end thereof, the final carbonconcentration profile desired for metallurgical reasons (profile ofmicrohardness--concentration of surface compressionconstraints--residual austenite content).

As indicated above, monitoring of the carbon surface concentration ofthe steel at a constant value may be obtained by controlling the inputfeed of the hydrocarbon into the reactor on the basis of a carbonbalance-sheet drawn up for the reactor acting as a thoroughly agitatedreactor. It is necessary, however, to determine with sufficient accuracythe moment when this maximum surface concentration is reached at the endof the transitory period which appears at the beginning of thetreatment.

An elegant solution to this problem consists in forming a layer of ironcarbide (called cementite) at the surface of the steel from thebeginning of the diffusion of the carbon by introducing into the supportmixture (nitrogen-hydrogen, for example) a suitable proportion of amixture of hydrocarbons, such as acetylene, methane, ethylene, ethane.This mixture permits the very rapid growth of a thin layer of cementiteat the surface of the steel. This thin layer, which is 1 micrometerthick, fixes the concentration of carbon at the cementite-steelinterface at the exact value of the saturation concentration of thecarbon in the austenite at the temperature considered. The diffusion ofthe carbon in the steel is therefore monitored by the existence of thislayer of cementite. It is this which supplies the carbon that diffusesin the steel. It suffices for the carbon consumed by the steel to becompensated for by the reconstitution of the layer of cementite which isfed with carbon from the hydrocarbons of the gaseous phase.

This process, which leads at the beginning of the treatment to the veryrapid formation (less than one minute) of a layer of cementite, permitsa self-monitored diffusion of the carbon.

The composition of the gaseous mixture which permits the rapid growth ofthe layer of cementite may be produced by a suitable addition of asingle hydrocarbon, propane, the decomposition of which upon contactwith the steel permits the desired gaseous mixture to be obtained,containing particularly and non-exclusively methane, ethane, ethylene,and acetylene.

Once the layer of cementite has been formed, its feed of carbon ispermanently regulated so that it compensates for the carbon consumed bythe steel. Dynamic transfer conditions are thus established between thegaseous phase, the layer of cementite, and the steel.

Under these conditions, for a given steel, the temperature is the onlyparameter which monitors the transfer of the carbon into the steel.

The adjustment of the final carbon concentration profile may necessitatethe partial transfer of the carbon from the solid to the gaseous phaseduring phase II. For this purpose, the gaseous mixture must contain aconstituent capable of combining with the carbon to form hydrocarbons,thus ensuring that the gaseous mixture has a decarburizing function.

The gaseous support mixture must therefore ensure three functions:

avoid any oxidation during all the phases where the flux of carbon fromthe gaseous mixture to the solid is nil;

form, with the products of decomposition of the hydrocarbon introducedduring the cementation proper, a mixture chemically capable of forming alayer of cementite permitting the transfer of the carbon into the steelto be ensured;

possibly ensure, after phase I, a function of partial decarburization ofthe surface of the steel.

These three functions may be obtained by using a gaseous support made upof a suitable mixture of nitrogen and hydrogen.

The principles set forth above can be demonstrated quantitatively withthe aid of thermogravimetric measurements with automatic treatment ofthe measurements and automatic control of the gaseous feeds.

It is found, for example, that the fluxes may be regulated at will,either at a constant value (curves a, b, c of FIG. 2) or by causing themto vary, so that the carbon surface concentration is constant (curve Th,FIG. 2).

Table I gives the instantaneous fluxes and the average fluxes forsurface concentrations equal to the saturation concentration of theaustenite. These fluxes are to be compared with those obtained in thecourse of conventional carbon monoxide treatments which reach 3.5mg/hr·sq.cm. at about 950° C.

It is found on the curve of FIG. 3, corresponding to case-hardening at850° C., that the algorithm for controlling the feed of propane permitsan absorption of carbon at constant surface concentration to be producedfrom the very first moments of the treatment. In fact, whatever thetemperature between 800° C. and 1100° C., the conditions of Dirichlet(constant concentration at the surface) are reached in less than oneminute, whereas case-hardening by carbon monoxide necessitates quitesome time (usually about one hour for the surface concentrations greaterthan 1% of carbon beyond 900° C.).

FIG. 1 shows schematically measurement means for gathering the datanecessary for controlling a treatment. An assembly 1 made up of afurnace of an analysis chamber is associated with a scale 2. Through avalve 3, the furnace is fed from a mass flow regulator 4 with gasescontained in bottles 5, in the present case three of them. A pump 6ensures the circulation of the gas and maintains the pressure in thefurnace. A thermocouple 7 monitors the temperature. Gas outlets 8, 9,and 10 permit the gases to be exhausted, the outlet 9 being connected toa chromatographic analysis apparatus, and the outlet 10 to a pressuresensor C and a pump P. A feed is possible at A. This installationpermits drawing up the material balance-sheet of the reactor, either byidentification of the chemical species formed and detection of theevolution of their concentration, or by continuous analysis of thegaseous mixture leaving the reactor.

The curves of FIG. 2, which correspond to certain particular cases, givethe quantities of carbon, in grams per square meter, which havepenetrated into the surface of the steel, as a function of the time ofexposure. The curves a, b, and c correspond to different operatingconditions, all comprising the sweeping of the steel at 1000° C. by agaseous flux in which the rate of flow of the support gas is 100 cc/min.The curve Th (theoretical curve) gives the quantities of carbon whichhave penetrated into the steel at the time t under comparablecondimtions when there is imposed upon the gaseous flux the conditionthat its flow is constantly such that it maintains the carbon surfaceconcentration at a constant value of cs=1.54%, which value correspondsto saturation.

The time needed to reach saturation in each of the conditions definedfor the flux by the curves a, b, and c are 0.8, 1.4, and 5.6 min.,respectively.

The curves of FIG. 3 illustrate one embodiment of the process of theinvention. Workpieces of a predetermined size are placed in a furnace at850° C. The curve Th gives, as in FIG. 2, the quantities of carbonwhich, according to the calculation, have penetrated into the steel as afunction of time, if the condition of surface concentration equal tosaturation is continuously maintained. The curve d reproduces arecording actually made during running of the process, while the curve egives, as a function of time, the rates of flow of propane in thegaseous mixture as they have been obtained by the control of theoperation. These rates are given in cc/min. The curve d is a consequenceof the curve e.

The enrichment phase ends after 24 min. The value of the carbon contentcorresponding to saturation is cs=1.08%. During the diffusion phase,which follows the enrichment, the feed of propane is reduced to zero.The quantity of carbon which penetrates into the steel first remainsconstant, then decreases.

It is seen that the curve d is situated slightly above the curve Th assoon as saturation has been obtained, i.e., after less than one minute.The rapid formation of a thin layer of cementite, which regulates thediffusion of the carbon in depth, is thus brought about.

FIG. 4 shows diagrammatically an installation for carrying out theprocess in a case where the data for controlling the gaseous mixturehave been stored or entered in the regulator in the form of analgorithm.

The furnace 11 comprises a muffle 12, an inner pot 13, a load ofworkpieces 14, and a turbine 15. It is equipped with heating elements16, a motor 17, and a cooling-water circuit 18 monitored by athermocouple 19. The gaseous flux entering at 24 and leaving at 25 ismade up of a carrier flux N₂ +5% H₂ and of a carbon-supplying flux C₃H₈. These fluxes come from bottles 20. They can attain a flow of 30lt/min. As a function of control parameters, valves 22 and 23 separatelyregulate the rates of flow of the bottles as a function of time. Theregulator 21 acts under the effect of data which can have been developedby the equipment of FIG. 1.

The graphs of FIGS. 5 and 6 relate to another embodiment of the process.The curves Th, d, and e of FIG. 5 have the same meanings as in FIG. 3.The indications on the y-axis are, respectively, masses of carbon havingpenetrated into the surface of the workpieces, in grams per squaremeter, and rates of flow of gas in liters per square meter and minute.The first scale relates to curves Th and d, the second to curve e.Case-hardening was carried out at 950° C. The carbon content atsaturation is cs=1.36%, which figure determines here the theoreticalcurve Th.

After an enrichment phase which lasts 24 min., as shown in FIG. 5, thediffusion phase takes place as illustrated in FIG. 6, which is aconventional graph reproducing depth concentration profiles. The curve fgives, in the conditions of the embodiment of FIG. 5, the profile of thecarbon concentrations at the end of the enrichment phase, i.e., after 24min. The curve g is a reading of the profile of the concentrations after22 min. of diffusion. The carbon content is about 0.8% at the surfaceand 0.1% at a depth of about 0.62 mm.

FIG. 4 shows the basic diagram of an industrial reactor for carrying outthis treatment. Numerous other reactor geometries are compatible withthis treatment; the usual apparatus for tempering steel either in aliquid medium (oil, water, etc.) or in a gaseous medium may obviously beadded to it.

FIG. 5 gives the recording of a typical treatment conducted at 950° C.with a first phase of 24 min. carried out at a constant surfaceconcentration of 1.38% carbon followed by a diffusion phase permitting asurface concentration of 0.8% carbon to be obtained at the end of thetreatment.

FIG. 6 gives the profiles of carbon concentration obtained.

Summing up, the process described presents the following importantelements:

I. It permits continuous and instantaneous checking of the flux ofcarbon transferred to the surface of a steel with the aid of a gaseousatmosphere ensuring the instantaneous and permanent checking of theconcentration of carbon at the surface of the steel.

II. It permits, besides the checking of the concentration of carbon atthe surface of the steel, the avoidance of any oxidation, even of themost oxidizable alloy elements of the steels. It is applicable to heattreatments of alloy steels, to the carburization sometimes calledcementation or case-hardening, and to the carbonitriding of steels.

III. It encompasses a process for accelerated cementation (orcarburization) of steels with the aid of a gaseous mixture containingone or more hydrocarbons permitting, under varied pressure conditionsand particularly at atmospheric pressure, the monitoring of the flux ofcarbon absorbed by the steel based on the monitoring of the feed ofhydrocarbons injected into the gaseous mixture.

IV. The accelerated case-hardening process permits the carbon surfaceconcentration to be maintained at the highest possible level compatiblewith the temperature of the steel for obtaining the maximum flux ofcarbon permitting enrichment of the surface with carbon during theshortest time possible.

V. This accelerated case-hardening process is characterized by a phaseof diffusion of the carbon into the steel, in the course of which thecarbon flux transferred from the gas to the steel is nil, thuspermitting the profile of carbon concentration to be adjusted based onthe surface and according to certain metallurgical criteria. In thecourse of this phase, the fine layer of cementite disappears since it isno longer fed with carbon by the gaseous phase.

VI. It is likewise characterized, if need be, by a phase of diffusion ofthe carbon into the steel, in the course of which a monitored carbonflux is transferred from the steel to the gaseous phase, thus permittingthe carbon concentration profile to be adjusted based on the surface andaccording to certain metallurgical criteria.

VII. In certain cases, the process permits carrying out the shortestpossible treatment in only two phases:

a phase of enrichment at maximum speed,

a phase of diffusion.

The process also permits slower running of the treatment leading to thesame carbon concentration profile with the aid of several alternatingphases of enrichment and diffusion intercombined in different ways.

VIII. It is possible to introduce nitrogen to the surface of the steelsimultaneously with the diffusion of the carbon. It suffices tointroduce a suitable quantity of ammonia into the mixture. Aself-monitored carbonitriding treatment is then achieved at the maximumrate of diffusion of the carbon and of the nitrogen.

IX. Use of a gaseous support mixture permitting oxidation of the steelto be avoided at any of the phases of treatment and made up inparticular of a mixture of nitrogen and hydrogen in suitableproportions.

X. Possibility of checking the carbon flux with the aid of themonitoring of the feed of propane introduced into the nitrogen-hydrogenmixture.

XI. The installation for carrying out the process may use all types ofbatch or continuous furnaces operating at low or high pressure, andparticularly at atmospheric pressure, provided that the gaseousatmosphere is thoroughly agitated.

XII. Monitoring of the carbon flux or of the surface concentration ofcarbon is obtained automatically by the coupled control of thetemperature of the steel and the hydrocarbon feed, and particularly ofthe propane.

An automatically controlled installation permits the case-hardening ofsteels at maximum speed at atmospheric pressure.

It completely avoids any formation, and hence any rejection, of carbonmonoxide and carbon dioxide.

                                      TABLE 1                                     __________________________________________________________________________    CALCULATION OF INSTANTANEOUS FLUXES AND AVERAGE FLUXES                        1000° C. Csal = 1.54% C                                                                 950° C. Csal = 1.36% C                                                              900° C. Csal = 1.20% C                   Time                                                                              Flux  Average flux                                                                         Flux  Average flux                                                                         Flux  Average flux                              in min.                                                                           Mg/h cm.sup.2                                                                       Mg/h cm.sup.2                                                                        Mg/h cm.sup.2                                                                       Mg/h cm.sup.2                                                                        Mg/h cm.sup.2                                                                       Mg/h cm.sup.2                             __________________________________________________________________________    0.10                                                                              56.49 112.99 37.89 75.78  24.78 49.57                                     0.30                                                                              32.62 65.23  21.88 43.75  14.31 28.62                                     0.50                                                                              25.27 50.53  16.94 33.89  11.08 22.17                                     0.70                                                                              21.35 42.71  14.32 28.64  9.37  18.74                                     0.90                                                                              18.83 37.66  12.63 25.26  8.26  16.52                                     1.10                                                                              17.03 34.07  11.42 22.05  7.47  14.95                                     1.30                                                                              15.67 31.34  10.51 21.02  6.87  13.75                                     1.50                                                                              14.59 29.17  9.78  19.57  6.40  12.80                                     1.70                                                                              13.70 27.40  9.19  18.38  6.01  12.02                                     1.90                                                                              12.96 25.92  8.69  17.38  5.69  11.37                                     2.10                                                                              12.33 24.66  8.27  16.54  5.41  10.82                                     2.30                                                                              11.78 23.56  7.90  15.80  5.17  10.34                                     2.50                                                                              11.30 22.60  7.58  15.16  4.96  9.91                                      __________________________________________________________________________     Csat = Limit of solubility                                                    Initial carbon = 0.1% C                                                  

What is claimed is:
 1. An installation for heat or thermochemicaltreatment on workpieces of a steel material, said workpieces having aninitial composition including an initial surface concentration ofcarbon, comprising:a furnace; a muffle within said furnace for receivinga load of said workpieces; a feeder for feeding said muffle with agaseous flux having a flux composition including a defined flux rate ofat least one hydrocarbon; a circulator for circulating said gaseous fluxwithin said muffle in contact with surface zones of said workpieces; aheater for heating said load and said gaseous flux and for maintainingsaid load and said flux at a defined temperature; a rate controller forvarying said defined flux rate of at least one hydrocarbon in said fluxcomposition during said treatment for carrying out at least one firstphase of said treatment, said at least one first phase comprising carbonenrichment of said workpieces; and a data processor for monitoring saidrate controller as a function of time the data processor including:control means for controlling the defined rate of said at least onehydrocarbon in the flux along said first phase, to provide an increasein the surface concentration of carbon up to a maximal valuecorresponding to a saturation state of said steel with carbon in asurface layer of said workpiece and to maintain this said surface layeras said saturation state until said workpiece is enriched by a definedamount of carbon, wherein said control means maintains the defined rateat a first constant value during a first time period of said firstphase, then instantaneously lowers the said rate and provides thenduring a second time period of said first phase, a regulated value ofsaid rate to maintain the surface concentration at said saturationstate.
 2. An installation according to claim 1 wherein said dataprocessor comprises data-storage means.
 3. An installation according toclaim 1 wherein said data processor comprises calculating means.
 4. Aninstallation according to claim 1 wherein said muffle and saidcirculator create thoroughly agitated reactor behavior of said furnaceat any defined temperature.
 5. An installation according to claim 1wherein said rate controller comprises valve means.
 6. An installationaccording to claim 1 wherein said rate controller instantaneously lowersthe rate after a time period of less than one minute from the beginningof said first phase.
 7. An installation according to claim 1 whereinsaid data processor monitors said instantaneous lowering of said rate ata defined time less than one minute after the beginning of said firstphase and monitors said end of said first phase approximately 24 minutesafter the beginning of said first phase.